Molecular mechanisms of prostate cancer metastasis to the bone are not well understood, and the ligand-receptor systems mediating this process are largely unknown. Proposed here is an integrated approach based on powerful platform technologies that will be used to dissect the molecular mechanisms of interaction between metastatic prostate cancer cells and the bone marrow. Recently established screening of random peptide phage display libraries in cancer patients enables quick isolation of ligands that can home to the vasculature of specific tissues. We propose to use bone marrow-homing peptides isolated in prostate cancer patients as leads for identification of human cell surface proteins mediating bone metastasis expressed by malignant prostate tumors. Peptide motifs homing to human bone marrow will be isolated using an enhanced in vivo phage display assay, parallel biopanning. The corresponding prostate cancer cell surface proteins containing the bone marrow-homing motifs will be identified using a specially designed bioinformatics platform. For several peptides validated to have high affinity and specificity for bone marrow, based on their testing in culture and in situ, we propose to identify the corresponding bone marrow receptors for the putative prostate cancer markers. For receptor identification, we will use a combination of previously established candidate receptor approach, conventional biochemistry, and the phage two-hybrid system: a recently developed assay for isolation of interacting proteins. As a result, this study is expected to identify at least one or two ligand-receptor pairs, which may be established or yet uncharacterized proteins that mediate the homing of metastatic prostate cancer cells to the bone. In the future, the identified molecules can be validated as potential biomedical targets for prostate cancer profiling and intervention by determining selectivity of their expression in the context of the disease. Upon optimization, this approach may be streamlined to systematically profile the molecular diversity of tumor cell-bone marrow nteractions for assessment of prognosis and/or for treatment of metastatic prostate cancer in individual patients.